A phased array antenna (PAA, also termed directive/electrically steerable antenna) is an array/matrix of antenna elements in which the relative phases or delays of the respective signals feeding the antennas are set in such a way that the effective radiation pattern of the array is reinforced in a desired direction and, at the same time, it is suppressed in undesired directions. The phase relationships among the antenna elements of the PAA may be fixed, or may be adjustable.
In basic PAA applications RF (analog) signals are delivered to/from the antenna elements through phase shift or time-delay devices configured to affect the desired radiation beam direction. In this way the angles of a directive beam can be instantly set in real time by electronically changing the phase shift of the RF signal of each antenna element. Better control over the radiation patterns can be achieved by simultaneously changing both amplitude and phase of the RF signals of each antenna element, also known as beamforming, used for achieving more general patterns of the formed beam, suppress side lobes, and to create radiation pattern nulls in certain directions.
In order to achieve accurate beamforming it is essential that all of the antenna elements of the PAA be amplitude and phase matched, or to a priori know the gain and phase differences of each antenna element of the array, which must be maintained in demanding environmental conditions over long time periods. Conventionally these goals been achieved using tight tolerance components, phase matched cables and/or factory measured calibration tables. However this is an expensive approach that offers little adaptation to the ambient environmental conditions.
The presence of amplitude and phase errors between antenna elements of the PAA cause distortions in the antenna radiation pattern in terms of beam pointing direction, sidelobe level, half power beam width and null depth. PAA calibration is typically achieved by tight tolerance design with factory determined calibration tables, radiative calibration utilizing internal and external radiating sources, and non-radiative dynamic calibration.
U.S. Pat. No. 6,346,910 describes an automatic array calibration apparatus which is capable of periodically calibrating beamforming offsets using internally generated calibration and test signals. The apparatus preferably includes a calibration signal generating unit which generates a continuous wave calibration signal which is input into a receiving channel as the input signal. I/Q signals are obtained from reception data channels which have been provided with the calibration signal. The apparatus also includes a loop back operation in which test signals are injected in transmission data channels, and are prepared for transmission at a transmission unit. The transmission signal is looped back to the receiving unit and I/Q signals are obtained from reception data channels supplied with the transmission signals.